This invention relates to valve gated injection molding and more particularly to an improved injection molding flow control mechanism.
The flow of melt to the cavity in a valve gated injection molding system is controlled by actuating a valve pin to seat its tip in the gate. In injection molding systems of the type disclosed in Canadian Pat. No. 1,029,162 which issued Apr. 11, 1968, this is done by applying a force to the other end of the valve pin by a pneumatically driven pivotal rocker arm. The rocker arm was supported by a pin extending through a transverse hole in the rocker arm with its ends received in opposing slots in the back plate. This had the advantage of reduced wear over previous knife-edge type fulcrums, but had the disadvantage that the hole, pin and back plate had to be machined to very close tolerances. Furthermore, in some mold configurations it is necessary that the runner passage bend as it passes through the back plate in order to go around this rocker arm mechanism. It is very desirable in injection molding technology that no sharp bends or corners be provided in the runner passage in order to avoid dead spots where build up and decomposition of the melt may occur. However, in these previous machined back plates, the runner passage was necessarily formed by drilling which resulted in sharp corners where bends occured.
More recently, the problems of the rocker arm pivot have attempted to be overcome by pivotting the rocker arm on the rounded end of a pivot pin received in a hole in the back plate and extending perpendicularly to the axis of rotation of the rocker arm. While reducing the number of interconnections from the previous structure, this still has the disadvantage that a very close tolerance fit is required between the pivot pin and the back plate. Furthermore, it will be appreciated that the operation of the rocker arm exerts a very considerable lateral thrust on the pivot pin, and it has been found that pins of this type supported at one end only made of material hard enough to withstand wear on the rounded end to the extent required, are not able to withstand this continual back and forth flexing pressure for the long periods of time required without breaking.